Chemical sensitization of photographic emulsions



CHEMICAL SENSITIZATION OF EMULSIONS Wesley G. Lowe, Jean E. Jones, and Harry E. Roberts,

Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey PHOTOGRAPHIC No Drawing. Application November 8, 1952, Serial No. 319,617

1 Claim. (Cl. 95-7) This invention relates to photographic emulsions and more particularly to chemical sensitizers therefor.

The digestion of washed photographic emulsions, free from silver halide solvents, is known to increase sensitivity without appreciable change in'grain size. 'This' process of chemical sensitization was'originally ascribed to formation of metallic silver nuclei by reaction of the silver halide with the gelatin or other vehicle. It is now known to be associated in gelatin emulsions primarily with the presence of reactive sulfur compounds which form silver sulfide. It has been demonstrated that sulfites can, under certain conditions such as relatively high silver ion concentration, act as chemical sensitizers and that the product of the reactionof sodium sulfite with silver bromide is probably metallic silver. Also the combined use of reducing agents such as arsenites withsulfur compounds for hypersensitizing has been disclosed. However, there are few specific compounds which have been described as chemical sensitizers other than those reacting to form silver sulfide. Stannous salts and alkali metal stannites, well known for their reducing properties, are described in the literature only ascausing fog f when used in connection with photographic emulsions" for purposes other than sensitizing.

It has been found that under the proper conditions stannous compoundsact as chemical sensitizers for photographic emulsions in a manner comparable to sulfur compounds and that this activity can be. superimposed on that of sulfur and natural gelatin sensitizers as well as optical sensitizers, particularly the latter, as disclosed in Carroll U. S. Patent 2,487,850.

It has now been found that when silver halide emulsions are sensitized with a sulfur sensitizing agent, a gold salt sensitizing agent and a stannous compound as a sensitizing agent, a speed'increase is obtained greater than that expected from the individual effects of the sensitizing agents and greater than that expected from the combined effects of any two of the three types of sensitizing agents.

Therefore, one object of our invention is to provide chemically sensitized emulsions of enhanced light sensitivity. Another object is to provide emulsions having sensitivity enhanced by the combined effect of sulfur, gold and stannous compound sensitizing agents. A further object is to provide the methods of augmenting cmulsion sensitivity utilizing the novel combination of sensitizing agents. Further objects of our invention will become apparent from consideration of the following description.

The objects of our invention are accomplished by the incorporation into silver halide emulsions of a mixture of (A) a sulfur sensitizing agent such as disclosed in Sheppard U. S. Patents 1,574,944 granted March 2, 1926, and 1,623,499 granted April 5, 1927, e. g., allyl isothiocyanate, thiocarbanilide, thiourea, sodium thiosulfate, dio-tolyl thiourea, thioacetamide and allyl thiourea, (B) a gold salt sensitizing agent such as disclosed in Waller and Dodd U. S. Patent 2,399,083, e. g., gold chloride,

potassium chloroaurate' or an alkali metal aurothiocyanate such as potassium aurothiocyanate and (C) small quantities of stannous compounds such as stannous chloride. p

' The exact type of stannous compound is not especially critical but stannous salts soluble in water but not readily decomposed are preferred, and because of the aerial oxi dation of alkali metal stannites, these are less preferred because of the difficulty of obtaining reproduceable.re-= suits. Therefore, acid solutions of stannous salts such as;

stannous chloride, stannous bromide, iodide, fluoride and sulfate are preferred. r

The stannous compounds are used in silver halide emul: sions in quantities of the order of from 1 l0-" to 44 l0- mol per mol silver halide. When using stannous salts such as stannous halides, we can use 'a range of concentration of from about 1' 10-' to 20 10" '1'nol per mol silver halide, preferably about 1 l0- to 9X10 mol per mol silver halide with the faster bromoiodide emulsions of the tyPe'show'n in the 'examplesbew low. When using alkali metal stannite solutions made. from the stannous salts, we prefer to use from about; l l0- mol per mol silver halide up to about 44x10 mol per mol silver halide, the higher quantities sometimesbeing required when the conditions of operation cause excessive aerial oxidation of the stannite solutions. Generally speaking, the lower ranges of concentration of stannous salts are used with the faster emulsions and up to about 20 10- mol per mol silver halide with slower fine-grained emulsions except when using stannite solutions. Other factors well known to those skilled in the art will be found to influence the optimum concentration of stannous salt to obtain the characteristics desired for a given photographicemulsion. The appearance of fog with a given emulsion and concentration of stannous salt is a good criterion to follow indicating that too much salt has been used. 1 p Thequantities of sulfur and gold salt sensitizing agents in the "silver halide emulsions are not especially critical: and can be varied from the optimum amounts indicated in the following examples. i The novel sensitizers are incorporated into emulsions digestion is complete. That is, the emulsion can first be sensitized with a sulfur sensitizing agent and a gold salt sensitizing agent followed by one of the stannous compounds or the addition'of the stannous compound can be made prior to addition of the gold salt. However, his

preferred to partially digest the emulsion in the presence of the selected sulfur and gold salt sensitizing agents and to finish digestion after addition of the stannous com pound since the greatest speed increases are obtained in this manner. One rather unexpected'feature of our invention resides in the fact that whileemulsions can be digested to optimum sensitivity in the presence-of sulfur;

. and gold sensitizing agents, further digestion in the'pres ence of the stannous compound does not cause any appreciable increase in fog.

The following example ilustrates means of carrying out our invention:

Example 1 A. In a positive type of silver bromoiodide emulsion were incorporated 20 milligrams of allyl thiourea per mol of silver halide and 2 milligrams of potassium chloroaurate per mol of silver halide and the emulsion was digested for 61 minutes at 62 C. A sample of the emulsion was coated onto film base.

B. To a sample ofemulsion A above were added 0.25 milligram of stannous chloride per mol of silver halide and the emulsion was digested for 20 minutes at 40 C before coating on film base.

, C. An emulsion was made as in B except using 0.5

Patented Apr. 24,- 1956;

millisrain oi stannous chloride per mol of silver halide. e qqatieas o emulsions A],

developed and evaluated sensitometrically with the results shown in the following table:

- developer of the followingcomposition was used for development of these emulsions:

l). A tine-grained positive type of silver 'bromoiodide gelatin emu si was. sen t ed n a con nt n l' er w th. a sul r n t e: suc a esc dabq e b new tastie 3. m nut M 0 6- scaled @040 n coa e a'film- E A. s m e of he same. ri n emuls o was en ii e n a coii t oaa manne w t he sulfur e i i used in D and a gold salt sensitizer such as described above, by heating for 30. minutes at 60 C., cooled to 40 and coated on film. Upon exposure, processing, and sensitometric evaluation, the emulsion was found to' have twice the speed of emulsion D. The combined effect of sulfur and gold salt sensitizing agents is thus illustrated.

Fl A S e of the sa e or na emu se n, D and E' was sulfur sensitized in a conventional manner with the same sulfur sensitizer and the same gold salt sensitizer used in E. by heating for 30 minutQSat 60 C cooled to 40 C. and /2 milligram of stannous chloride. per mol of silver halide was added, The emulsion was then; held. for 20' minutes at 40 C. and coated on iilrn, Upon exposure and processi g, this emulsiqn was found to have ,twice the speed of emulsion 1;, and four times, the speed of emulsion D. This example, aswell asvExample l illustrates enhancing the speed of a sulfur, and

gold sensitized emulsion byv digesting in thepresence of a stannous compound.

In processing the, emulsions ab ove, development was carried out in a developer of the followin Composition: Water about 125715. cc 750 p-Methylantinophenolsulfato ......grams- 0.3:; lgly lroqi no e "fla -r 0--- -0 Sodinn tsulfite (desiceated), do- 38:0 Sodiuggbisulfite a "doe-" 1.2,. n u H In carbonateddesiccated) doe-n. 19.0 39 .l @;b ?QP d=---.-- dise se. .d on: W te -to. ,-0 t

and C were exacted...

In the manner of the above examples, other sulfur sens iias. assets, 5915. v salt sensi izing again an stannous compounds can be used. When alkali metal stannites are employed to augment sensitivity of sulfur and gold sensitized emulsions, they are simply prepared by dissolving stannous chloride in water and adding suflioient sodium hydroxide to dissolve the oxide first formed. While the, above example shows employing digestion after addition of the stannous compound, it is usually not necessary to heat the emulsion after the stannous compound has-been added inasmuch as the stannous compounds react very rapidly to enhance emulsion sensitivity.

In the manner of the above examples, many types of silver halide emulsions may be. treated with the novel combination of sulfur, gold and stannous sensitizing agents. The novel sensitizers may, in general, be employed in hydrophilic colloid emulsion vehicles; for example, gelatin, polyvinyl alcohol, partially hydrolyzed s ll osc ster suc as cellu os acetate and copo y of polyvinyl alcohol, The sensitigersare suitable for the well lrnown types of; silver chloride, silver bromide and silver iodide. emulsions and emulsions containing mixtures of these halides, containing the usual emulsion addenda such as nntifoggants, spreading agents, coupler qomnoun a. u h. emulsions, are suitable to use. inv for ming single wellas multilayer film useful in. color HhQiQBYflP y w ich c stomar y ncl on a s pp rt two or; more emulsion layers sensitized; to different regions of hsv sih e sp ctrum and m y o y not contain coupler compounds. According to our invention such color films carry at least one emulsion layer containing the mentioned combination of sensitizing agents.

Qur.- inventionhaving been; described, we would have it understpqd that the preceding, description is by way of example only: and that our; invention is to, be taken as. limited onlyby-the scopeof the appended claim.

What we, claim is A ligh t sensitive, photographic emulsion comprising silver halide sensitized, with a sulfur sensitizing agent, a gold salt sensitizing agent, and: from 1x10 to 2O 10- mol. of stanuous chloride per mol of silver halide.

References Cited. inthe file of this patent UNITED STATES PATENTS 2,399,083 Walker et al Apr. 23, 1946 2,487,850 Carroll Nov. 15, 1949 2;,540,Q8'6. Baldsiefen et al, Feb. 6, 1951 2,597,856 Damschroder May 27, l952 

